Hot melt adhesives (HMAs) are a form of thermoplastic adhesive that is designed to be applied in the molten state. The glue is tacky when hot and solidifies in a few seconds to about one minute. HMAs may also be applied by dipping or spraying.
In industrial use, HMAs provide several advantages over solvent-based adhesives. Volatile organic compounds are reduced or eliminated, and the drying or curing step is eliminated. HMAs have a long shelf-life and usually may be disposed of without special precautions. Some of the disadvantages include thermal load of the substrate, limiting use to substrates not sensitive to higher temperatures, and loss of bond strength at higher temperatures, up to complete melting of the adhesive. These may be mitigated by using a reactive adhesive that, after solidifying, undergoes further curing, for example, by moisture (e.g., reactive urethanes and silicones) or with ultraviolet radiation. Additionally, some HMAs may not be resistant to chemical attacks and weathering, and HMAs do not lose thickness during solidifying, while solvent-based adhesives may lose up to 50-70% of layer thickness during drying.
HMAs formulations of the prior art include styrene block copolymer (SBC, aka rubber) based formulations, acrylic-based (acrylics) formulations, silicone-based formulations, and metallocene polyethylene- and metallocene polypropylene-based formulations.
Amorphous poly alpha olefin (APAO) polymers are compatible with many plasticizers, tackifiers, waxes, and polymers; they find wide use in many adhesive applications. APAO HMAs have good acid resistance, moderate heat resistance, and light and UV resistance due to their saturated hydrocarbon nature; are tacky, soft, and flexible; and have good adhesion to multiple substrates and longer open times than crystalline polyolefins. APAOs tend to have lower melt viscosity, better adhesion, longer open times, and slower set times than comparable ethylene vinyl acetate (EVA) copolymers. Some APAOs may be used alone, but they are often compounded with co-adjuvants such as tackifiers, waxes, and plasticizers (e.g., mineral oil, polybutylene oil, naphthenic oil). Examples of APAOs include amorphous (also known as atactic) propylene (APP, CAS #9003-07-0), amorphous propylene/ethylene (APE, CAS #9010-79-1), amorphous propylene/butene-1 (APB, CAS #29160-13-2), amorphous propylene/hexene-1 (APH, CAS #25895-44-7) copolymers, and amorphous propylene/ethylene/butene-1 (APEB, CAS #25895-47-0) and amorphous propylene/butene-1/hexene-1 (APBH) terpolymers. APP is harder than APE, which is generally harder than APB, which is generally harder than APH, in accordance with decreasing crystallinity. And in accordance with their decreasing crystallinity, APP has higher tensile or mechanical strength than APE, which has generally higher tensile or mechanical strength than APB, which has generally higher tensile or mechanical strength than APH. Due to their lower molecular weights as compared to other polymers such as SBCs, acrylics, or many metallocene polyethylenes and polypropylenes, at typical application temperatures of 375 degrees Fahrenheit, APAOs exhibit a high degree of substrate wetting, which is a very desirable HMA property. However, APAOs show relatively low cohesion, as the entangled polymer chains have a fairly high degree of freedom of movement. Under mechanical load, most of the strain is dissipated by elongation and disentanglement of polymer chains, and only a small fraction reaches the adhesive-substrate interface. Cohesive failure rather than adhesive failure is therefore amore common failure mode of APAOs.
APAOs are produced by the (co-)polymerization of α-olefins, e.g. ethylene (CAS #74-85-1), propylene (CAS #115-07-1), butene-1 (CAS #106-98-9), or hexene-1 (CAS #592-41-6), with Ziegler-Natta catalysts. The (co-)polymers have an amorphous structure which makes them useful for the production of HMAs.
The present embodiments meet these needs.